1. Field of the Invention
The present invention relates to a thermal processing method and a thermal processing apparatus for thermally processing a substrate such as a semiconductor wafer or a glass substrate (hereinafter simply referred to as “substrate”) by irradiating the same with light.
2. Description of the Background Art
In general, a thermal processing apparatus such as a lamp annealing apparatus using a halogen lamp is employed in an ion activation step for an ion-implanted semiconductor wafer. This thermal processing apparatus heats the semiconductor wafer to a temperature of about 1000° C. to 1100° C. (annealing), for example, thereby carrying out the ion activation step for the semiconductor wafer. This thermal processing apparatus increases the temperature of the substrate at a rate of about hundreds of degrees per second through the energy of light emitted from the halogen lamp.
However, it has been proved that the profile of ions implanted into the semiconductor wafer is rounded, i.e., the ions are thermally diffused when the thermal processing apparatus heating the substrate at the rate of about hundreds of degrees per second carries out the ion activation step. If this phenomenon takes place, the ions implanted into the surface of the semiconductor wafer are diffused also when the same are implanted in high concentration. Therefore, the ions must disadvantageously be implanted beyond necessity.
In order to solve the aforementioned problem, Japanese Patent Application Laying-Open Gazette No. 59-169125 (1984), No. 63-166219 (1988) or the like proposes a technique of irradiating the surface of a semiconductor wafer with flash light through xenon flash lamps or the like thereby increasing the temperature of only the surface of the ion-implanted semiconductor wafer in an extremely short time of not more than several milliseconds. When heating the semiconductor wafer with the xenon flash lamps for an extremely short time, the ions are not diffused due to a shortage of time and hence the thermal processing apparatus can carry out only the ion activation step without rounding the profile of the ions implanted into the semiconductor wafer.
However, the conventional thermal processing apparatus having such a structure has the following problem: The thermal processing apparatus employing xenon flash lamps is provided with a plurality of bar-shaped or similar xenon flash lamps. When the thermal processing apparatus approaches the plurality of xenon flash lamps to the semiconductor wafer, it follows that illuminance is increased in portions of the wafer surface located immediately under the lamps beyond that in the remaining portions, to damage in-plane uniformity of temperature distribution. However, the thermal processing apparatus using the xenon flash lamps having an extremely short heating time cannot employ a method of maintaining in-plane uniformity of illuminance distribution by rotating the wafer dissimilarly to the conventional lamp annealing apparatus using a halogen lamp.
While a reflector is arranged along with the lamps in order to effectively utilize light emitted from the lamps, illuminance reflected by this reflector also has a light intensity distribution on the wafer surface. When the lamps emit light so that the amplitudes of the light intensity distribution of reflected light and a light intensity distribution of irradiation directly applied to the wafer from the lamps substantially coincide with each other, it follows that in-plane uniformity of temperature distribution is further damaged.